“Floating screed” asphalt finishing machines, or pavers, have provided an efficient and economical method of coating an old or new roadway with a compacted layer of asphalt aggregate for many years. Floating screed pavers are generally known to those skilled in the art, as reflected by the disclosure contained in U.K. Patent 1,054,151 to the Blaw-Knox Company. Such a paver typically comprises a self-propelled traction unit, or tractor, having a hopper at its front end for receiving paving material, such as asphalt aggregate, from a dump truck.
A conveyor system on the machine transfers the paving material from the hopper rearwardly for distribution in front of a floating screed. Transversely arranged screw augers positioned at the rear end of the traction unit assist in moving the paving material in a lateral direction with respect to the direction of movement of the paver, so that a relatively uniform volume of paving material is distributed across the portion of the roadbed in front of the floating screed.
The screed is commonly operated so as to “float” by virtue of being connected to the forwardly moving machine by means of pivoted leveling arms or tow arms. With forward movement, the screed physically levels any paving material lying higher than a predetermined height above the roadway surface, leaving a generally uniform thickness of such material. This function is enhanced by inclining the bottom surface of the screed so that its forward edge is higher than its rear edge, thereby providing a smaller area between the screed and the roadway and a large dragging surface at the rear of the screed. The angle defined between the bottom surface of the screed and the roadway surface is called the “angle of attack.” The screed also compacts the dragged paving material in order to provide a uniform, smooth, durable pavement surface. The screed is often mounted to vibrate against the pavement material to assist in spreading and compacting the material.
The leveling arms of the screed are attached to the paver fraction unit at a “tow point.” In early pavers this point was a simple fixed pin connection. As a result, the thickness of the resulting paved mat could only be controlled by means of altering the screed angle of attack. Later paver designs allowed the tow point to be moved vertically, causing a corresponding movement in the leveling arms and screed. This arrangement accommodated changes in the grade of the road surface by automatically fine tuning the initial setting of the screed angle of attack, thereby controlling the pavement mat thickness.
The tow point connection between the traction unit and screed units affects two aspects of paver operation, in particular. It is generally preferred to add asphalt to the paver while the paver is still moving in order to avoid joints or dips and valleys in the resulting asphalt mat caused by stopping and starting movement of the machine. Thus, dump trucks containing the asphalt material are usually positioned ahead of, and in the path of, the paver. As the paver approaches the truck, or as the truck is backed towards the paver, push rollers on the front of the paver engage or are engaged by the dump truck's rearmost tires. At the moment of impact between the truck and paver, the paver is usually urged backwards, particularly if the truck has been backed into it. A screed can damage the surface of the newly laid asphalt mat, causing bumps and valleys, if the screed leveling arm is rigidly connected to the traction unit as a paver is urged backwards.
Further, the distance between the screed and the transverse auger of the traction unit is usually adjusted to reflect the largest aggregate particle size being laid. In many asphalt mixes, especially those with high percentages of recycled asphalt, it is desirable to minimize the gap between the transverse auger and the front plate of the screed to minimize the buildup of hardened “dead” material, thereby reducing tearing of the asphalt mat and improving the quality of the end product. This gap is adjusted most easily at the tow point.